Rendering
Render Reset Simulation
Normally when the rendering is started the TP simulation will be reset and re-calculated, because of different settings for viewport or rendering.
When you only want a test rendering at a specific time as sample with time-consuming operators like the Flow or Smoke TP simulates the whole sequence to the frame before the rendering started.
Now with Render Reset Simulation off the rendering starts immediately with the TP simulation status as it is at that time, so you get faster rendering to adjust materials or the Smoke rendering interactively.